Experiments and Observations Based on the Finding that Scleractinian Corals Secrete Calcite Instead of Aragonite in Cretaceous Seawater

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ABSTRACTThe chemistry of seawater has changed over the past half billion years in response to changes in rates of volcanism in the deep sea. The proposed research relates in particular to changes in the magnesium/calcium ratio of seawater induced by changes in the rate at which hydrothermal activity releases calcium from Earth's mantle into the ocean and consumes magnesium from seawater. Correlations between these changes and the mineralogy of important reef-building and limestone-forming organisms suggest that the magnesium/calcium ratio of the ocean has exerted a strong influence over biomineralization by marine life.At magnesium/calcium ratios below 1, calcium carbonate, the primary constituent of limestone, precipitates inorganically in the form of the mineral low-magnesium calcite. At ratios between 1 and 2, high-magnesium calcite precipitates instead, and at ratios above 2, high-magnesium calcite and/or aragonite (a mineral with a different crystal structure) precipitate. At the present time submarine volcanism is relatively weak, so that the magnesium/calcium ratio of seawater is high (5.2). It can be hypothesized that corals flourish as reef builders today because they secrete aragonite, which is favored by the high ambient magnesium/calcium ratio. During Late Cretaceous time, about 100-65 million years ago, corals yielded their role as dominant reef builders to rudist bivalves, which secreted shells consisting largely of calcite. At that time, the magnesium/calcium ratio of seawater had dropped to the vicinity of 1, favoring calcite precipitation. Corals did not resume their dominant reef-building role until Oligocene time, when the magnesium/calcium ratio was again rising into the aragonite domain.The research now proposed will test the effects of the magnesium/calcium ratio on coral skeletal growth through laboratory experiments. Four species of corals will be cultured in seawaters having 7 different magnesium/calcium ratios, including those of modern and Late Cretaceous seas. Skeletal growth rates and skeletal mineralogies will be compared for the various treatments. Relatively low growth rates in "Cretaceous" seawater would suggest that the low magnesium/calcium ratio of Late Cretaceous seas accounted for the weak reef-building role of corals in those seas. A partial or complete shift in "Cretaceous" seawater to precipitation of calcite instead of aragonite would demonstrate an even stronger influence of seawater chemistry on biomineralization.